This invention relates to an external magnetic field generating device for use in magneto-optic recording.
The magneto-optic system, which is one type of optical disk device capable of erasable recording, uses the following principle: A magnetic film is heated with a laser spot and a magnetic field of several hundred oersteds is applied, magnetizing the magnetic film, when the heat is removed, in the direction of the external magnetic field. If one direction of magnetization signifies "recording" or bit "1", the opposite direction signifies "erasure" or bit "0".
FIG. 1 shows a first example of a conventional external magnetic field generating device for the implementation of the above-mentioned magneto-optic system. In the figure the external magnetic field generating device is comprised of permanent magnet 2 mounted on a magneto-optic disk 1 for recording, and an actuator (for example an actuator using a magnet etc.), not shown, which rotates permanent magnet 2 about axis 3. Permanent magnet 2 is in the form of a rod which elongates in the direction of the radius of the disk 1 and is magnetized N and S in its width direction. The magnetic field component of this magnet which is perpendicular to the disk contributes to the magnetic inversion of the magnetic film of disk 1. In other words, as shown in the figure, by rotating with an actuator permanent magnet 2, on which are arrayed magnetic poles S and N in a direction perpendicular to disk 1, 180.degree. about axis 3, magnetic recording and erasure may be performed.
FIG. 2 shows a second example of a conventional external magnetic field generating device. This external magnetic field generating device is, as shown in the figure, comprised mainly of an electromagnet 4. The electromagnet 4 has an elongated form and is mounted to extend in the direction of the radius of the magneto-optic disk 1 for recording. Protruding teeth 5, 6a and 6b are formed on its bottom face. The coil 7 is wound around the central tooth 5 and the magnetic field is generated by sending an electric current through the coil 7. The magnetic field so generated enters the teeth 6a and 6b, on either side of the central tooth 5, from the central tooth 5 and the component of this magnetic field which is perpendicular to the disk becomes the external magnetic field, performing recording or erasure on the magnetic film. The inversion of the magnetism of the magnetic field is accomplished by reversing the direction in which the electric current is sent.
However, the first and second examples of conventional external magnetic field generating devices described above contain problems, as described below.
In the first example of the external magnetic field generating device, the permanent magnet must be mechanically rotated by some external force, in other words an actuator, when inverting the magnetism of the magnetic field. In order to rotate a magnet which has a large amount of inertia, ten to several hundred milliseconds of operating time is required, giving various limitations to its use.
Also, in the second example, in order to wind the coil 7 of FIG. 1 the requisite number of times the tooth 5 must be high. The teeth 6a and 6b on either side of the tooth 5 are also as high as the tooth 5. Therefore, because of the dimension in the height direction of the teeth, this example has the drawback that the entire external magnetic field generating device becomes large. And this drawback has greatly retarded the miniaturization and thinning of magneto-optic disk devices.